Snowmobiling is a popular winter activity in cold climates. The steering of snowmobiles is conventionally accomplished by a pair of ground engageable skis that are mounted to the front suspension system of the snowmobile and are turned in the direction in which the snowmobile is to be propelled. The skis 20 and front suspension system 12 of a conventional snowmobile 10 are shown in FIG. 1A. The front suspension system 12 to which the skis 20 are attached includes a pair of A-arms 14, a pair of shock absorbers 18, and a pair of coil springs 16 disposed around the shock absorbers 18. The primary purpose of the coil springs 16 has traditionally been to support the weight of the snowmobile 10 and maintain its ride height, while the primary purpose of the shock absorbers 18 is to dampen the movement of the springs 16.
Most conventional suspension systems are adjustable to some extent. The vast majority of adjustments to the springs 16 are geared toward setting ride height, while the vast majority of adjustments to the shock absorbers 18 are geared toward changes in trail conditions or personal feel preferences.
As shown in FIG. 1C, a rigid chassis 30 links a snowmobile's front suspension 12 and rear suspension 40. Adjustments made to the front suspension 12 will transfer energy to affect the rear suspension 40. Lowering the front suspension 12 will unload the rear suspension 40 causing it to top out. Raising the front suspension 12 will cause a low rear ride height, simulating the need for more spring preload in the rear suspension 40. Accordingly, once the desired height is reached for the front suspension 12, and the rear suspension 40 is properly adjusted, it is preferable to avoid changing this height to account for different trail conditions or to tune the ride of the snowmobile.
Tuning adjustments are typically made to shock absorbers 18 and most may be adjusted to stiffen or soften the ride. Increasing the stiffness of the shock absorbers 18 will reduce contact with the trail and steering ability. Conversely, decreasing the stiffness will enhances contact with the trail and steering ability, but increases the risk of harsh bottoming out on rough terrain and body roll during cornering. Accordingly, as general rule, the shock absorbers 18 should be adjusted to be as soft as possible to allow for controlled steering while preventing excessive body roll and the bottoming out of the springs 16 when the skis 20 hit rough terrain. Given this fact, it is also preferable to avoid making tuning adjustments to the shock absorbers 18 once a desired ride is found.
Front suspension springs 16 are of typically of two types: linear and progressive. Linear springs 16 are the traditional type and they allow for comfort in groomed trail conditions. However, as shown by the dashed line in FIG. 2, these springs compress at a linear rate and are more prone to bottoming out under heavy suspension load.
Progressive springs 16 were developed to address this issue. A progressive spring 16 compresses at a linear rate until a certain load or displacement, after which it compresses at a non-linear rate, requiring far more force to be applied before bottoming out occurs. This is shown by the dashed line in FIG. 3, which plots the spring rate for a typical progressive spring. This allows for comfort in groomed trail conditions but progressively stiffens the ride as these springs compress for better cornering and handling. However, nearly all factory installed springs are linear springs and progressive springs are typically fairly costly aftermarket items purchased by racers and other high performance users and, therefore, most snowmobiles have linear springs. Further, progressive springs have a linear rate cutoff and a set non-linear rate and once installed, are not adjustable. Finally, many progressive springs achieve their progressive nature through the variability of distance between spring turns and have a very small distance between some turns, effectively limiting the amount of travel of the spring before bottoming occurs.
Therefore, there is a need for a way to tune a snowmobile's ride without adjusting the height of spring 16, stiffness of shock absorbers 18 or rear suspension 40, that allows a linear spring to function like a progressive spring, and that allows the performance of the spring to be quickly and easily changed.